Understanding the unique S-scheme charge migration in triazine/heptazine crystalline carbon nitride homojunction

Understanding charge transfer dynamics and carrier separation pathway is challenging due to the lack of appropriate characterization strategies. In this work, a crystalline triazine/heptazine carbon nitride homojunction is selected as a model system to demonstrate the interfacial electron-transfer mechanism. Surface bimetallic cocatalysts are used as sensitive probes during in situ photoemission for tracing the S-scheme transfer of interfacial photogenerated electrons from triazine phase to the heptazine phase. Variation of the sample surface potential under light on/off confirms dynamic S-scheme charge transfer. Further theoretical calculations demonstrate an interesting reversal of interfacial electron-transfer path under light/dark conditions, which also supports the experimental evidence of S-scheme transport. Benefiting from the unique merit of S-scheme electron transfer, homojunction shows significantly enhanced activity for CO2 photoreduction. Our work thus provides a strategy to probe dynamic electron transfer mechanisms and to design delicate material structures towards efficient CO2 photoreduction.

***Originality and significance: If the conclusions are not original, please provide relevant references. On a more subjective note, do you feel that the results presented are of immediate interest to many people in your own discipline, and/or to people from several disciplines?*** The results are potentially interesting and relevant to a broad audience of people working with photosensythysed systems. ***Data & methodology: Please comment on the validity of the approach, quality of the data and quality of presentation. Please note that we expect our reviewers to review all data, including any extended data and supplementary information. Is the reporting of data and methodology sufficiently detailed and transparent to enable reproducing the results?*** The methodology used in the DFT calculations is not clear, and therefore the criteria of reproducibility is not met. It is not clear how many systems were modeled: TCN alone, HCN alone and the TCN/HCN homojunction? Or only the latter? Are Li+ and Cl-included in the simulation box? Please state clearly this information in the method section ( a Table might be useful) and represent all of the model systems clearly (see examples above) either in the main text or in the supporting. 2) the separation between the periodic structures along the z-axis is set to 15 Å. This value is smaller than that of 20 Å used in other investigations to avoid spurious interactions between periodic image (see e.g. NATURE COMMUNICATIONS | (2020) 11:4613; J. Phys. Chem. C 2018, 122, 7712−7719; Applied Catalysis B: Environmental 2020, 268, 118381). While the value of 15 Å has been used also in other calculations, my concern here is due to the presence of ions, which could make the interaction between layers be stronger (i.e. longer ranged) than for other systems were neutral layers are modeled. I suggest the author to check whether the separation may affect the results and the conclusions, or if there are other evidences that indicate that the use of this separation does not alter the results on similar systems.
3) What do the authors mean with "simulated scanning tunneling microscopy (STM) images" (lines 125-127, 573, 577) and what they are used for. Do the authors refer to a STM images calculated as in reference: Scientific Data volume 8, Article number: 57 (2021)? If so, I have the following concern. If I understand correctly, in the present manuscript no STM experiment is performed. Could the author comment why the simulated STM images are useful in the present investigation if there is no experimental reference? It is not obvious how a simulated STM image can "unveil triazine and heptazine structures as the basic units of TCN and HCN": if the structures were used as input for the simulations of the STM spectra, what else could be expected? 4) Please specify how the work functions were calculated. 5) Charge density maps: the analysis of this property has an important role in this investigation to supports the proposed S-scheme mechanism. However, the data presented are not clear, and also the discussion and conclusions reached with the presented data are not convincing in the present form. In particular, in Sup. Inf. Fig. 9: the pictures (a) and (b) are taken from different points of view and are difficult to compare. Exactly the same orientation should be used to allow the reader to make the comparison and verify the conclusion of the authors. Fig (c) and (d): there are literally infinite possible planes on which the charge density can be represented. Depending on which plane is taken completely opposite conclusion can be (in principle) drawn. Why the authors draw their conclusions only one particular plane? How the conclusions drawn on a particular plane can be considered general? 6) Mulliken charges are known to have several drawbacks, and other schemes for calculating the charges partitioning among atoms have been developed. Considering that the authors are proposing a strategy of general interest, it could be useful if the they can comment on the validity of this aspect of the computational approach they are proposing.  , the atom scheme coloring is a bit  confusing, it would be easier for the reader to always have gray color for C atoms and blue for Nitrogen,  as used in other Figures, such as supporting Fig 9. Same problem in Fig. 5c, it is very difficult to visualize the structure with the representation used here for C and N, and it is not clear why some yellow balls (nitrogen) are not linked to any carbon atoms.. Also, I find confusing the use of two different molecular representations in the same figure (in fig 5a and b is ball and stick with different colors from the vdw representation in fig 5c). • Fig 2f: it would be easier to analyse if the y axis values would be aligned among the three subfigures. The quality of the figure is low, both in term of resolution and in terms of "collaging" quality (alignment and styles of the graphics). : what the pink and cyan balls are representing? 9) Other comments on the text: • Line 129: "TH1:4, i.e. the ratio of heptazine/triazine is 1:4" is this phrase correct? I find confusing that the order of the initials in the acronym is opposite to the ratio of the components to which the initials refers. It would be clearer as HT1:4 if the ratio H/T is 1:4. • Line 223-224: here the authors comment on the electrostatic potential, but I cannot find where the results on this property are reported. ***Appropriate use of statistics and treatment of uncertainties: All error bars should be defined in the corresponding figure legends; please comment if that's not the case. Please include in your report a specific comment on the appropriateness of any statistical tests, and the accuracy of the description of any error bars and probability values.*** No statistical analysis is reported, the possible errors are not commented. ***Conclusions: Do you find that the conclusions and data interpretation are robust, valid and reliable?*** As detailed above, my main concerns are related to the reported computational data. *** Suggested improvements: Please list additional experiments or data that could help strengthening the work in a revision.*** See section data and methodology. There are some errors/typos in the text. A careful read through for checking grammar errors is suggested. ***References: Does this manuscript reference previous literature appropriately?*** YES *** Clarity and context: Is the abstract clear, accessible? Are abstract, introduction and conclusions appropriate?*** In general, yes with the exception of what noted above. ***Inflammatory material: Does the manuscript contain any language that is inappropriate or potentially libelous? *** NO, it does not *** Springer Nature is committed to diversity, equity and inclusion; please raise any concerns that may in your view have an impact on this commitment.*** Does not apply *** Please indicate any particular part of the manuscript, data, or analyses that you feel is outside the scope of your expertise, or that you were unable to assess fully.*** My expertise is mainly in the computational field. I have no experience with KPFM.
Reviewer #3 (Remarks to the Author): The paper describes an procedural analysis on how and why a recently described all organic heterojunction, PHI-PTI, generates its record high efficiencies in overall water splitting and CO2 reduction. Contrary to previous work which generates its solid-state heterojunction photochemical material by simultaneous cocrystallization and salt melt etching, the present approach is based on independent crystallization and later electrostatic co assembly, which gives a larger structural size enabling to follow photogenerated charges, e.g. by surface potential measurements. Wioth a combination of data, the presence of a S-heterojunction as well as the relative role in oxidation and reduction processes was determined. This makes the paper interesting and worth the ctation by a very large, very competetive community. This also brings me to my very important point of criticism dealing with correct citations within the ultracompetetive asian market of artificial photosynthesis. To make a long analysis short: the manuscript essentially cites one of the two schools, even overscribing priority of the other school by citing later secondary papers of the first school. Being not involved in this specific fight for leadership, I find it nevertheless non acceptable. I seriously and kindly ask the authors to consider the rules of fairness and respect. Triazine-Heptazine-Based Copolymers with Apparent Quantum Yields of 60% at 420nm for Solar Hydrogen Production from "Sea Water", as well as DOI0.1002/anie.201706870). The same group later expanded this system to photochemical CO2 reduction, also with record high conversion values (DOI10.1002/anie.201811938). The heterojunction concept as well as driving both artificial photosynthesis reaction on the reduction side with record high efficiences was thereby known before.
In digital age, such things are easy to find out, and I could go on with a localization of where most citations go to and what else was ignored for that, and I would do in a second round if the authors do not get now what principles in scientific publishing are about. It is to my opinion indeed a very good, careful paper, and unbalanced recitation of the market can only ruin that impression.

Response to reviewers
We thank the editor and referees for their time and valuable comments on our manuscript. We have studied the reviewer's comments carefully and have made conscientious revisions. In the following, we provide a point-by-point response to their comments.

Reviewer #1 (Remarks to the Author):
This manuscript demonstrates the so-called homojunction between two allotropes of carbon nitride and investigated the charge transfer behavior between the two components. The study of the charge carrier transfer at the interface is of significant importance to deeply understanding the photocatalytic mechanism. The main assumption of this submission is attractive, but more solid characterizations are still lacked. However, there are no such characterizations (such as TEM) to provide direct evidence. More detail information at the interface is highly desired to support the main assumption of the manuscript. It is widely considered that the weak electrostatic interaction between TCN and HCN only generate nanocomposites rather than junctions. In addition, the peaks of HCN in this work appear right-shifted and the peak shape becomes sharp around 27° compared to that of the BCN, which is consistent with the literature reports 4,5 , and the transmission electron microscopy reveals clear lattice stripes, which is sufficient evidence for the high crystallinity of HCN. In addition, the characteristic peak at 13.1° related to the in-plane stacking mode (100) is very weak, which is caused by the curling folds of the ultrathin nanosheets 6 .

Response
In fact, the broad sense of carbon nitride material contains g-C3N4, PTI, PHI, etc. At present, many people in the field confuse triazine-based g-C3N4, heptazine-based g-C3N4, PTI, PHI, with each other, and in fact, they are completely different substances. Strictly speaking, we often refer to carbon nitride materials as g-C3N4, but since this kind of material was initially predicted to exist through a computational simulation, the corresponding crystal structure model was simultaneously assumed for the perfect crystallization of this material and adopted till now. However, in fact, after more than a century of development, this material is still not perfectly crystalline when synthesized experimentally 7 . In 4 order to improve the crystallinity of carbon nitride materials, researchers have tried various methods such as high-temperature and high-pressure methods, ultrasonic methods, and molten salt methods 8 . Among them, the high crystallinity of materials such as PTI and PHI synthesized by molten salt method has attracted much attention. Therefore PTI and PHI were called crystalline carbon nitride in the early days. Strictly speaking, PTI and PHI are derivatives from the process of synthesizing crystalline carbon nitride, and that are not identical to g-C3N4. Here we need to point out that trace metal salt residues are unavoidable in the preparation of this material, but do not affect the structure and properties of this As noted by the reviewer, previous studies by Lotsch and Antonietti claimed that Li + and Clions are presented in PTI/Li + Cl -, in this work, after comparison with the XRD pattern of PTI, the TCN in this work is PTI, while HCN is not K-PHI. 4. Connected the above question, the acid treatment of HCN would lead to ion exchange (K + was replaced by proton). Elemental analysis of the samples before and after acid treatment are involved.

Response: The content of K and Li was obtained from ICP-OES (PE Avio 200) and the content of H was obtained from O, N and H co-meter (LECO ONH836).
As shown in Fig. R2, after acid treatment, the content of metallic elements decreases significantly, indicating that acid washing removes some of the metal salts. In fact, our group has studied in detail the effect of acid treatment to eliminate K ions on the photocatalytic activity of crystallized carbon nitride in 6 our previous work 10     8. In Figure S15, the CO2 reduction activity dramatically decreased only after few hours' reaction. Is this derived from the weak interaction between the two components?
Response: If there is a weak interaction between TCN and HCN, then the photocatalytic activity will not be significantly enhanced in the first few hours, so we speculate that the decrease in activity is not related to the interaction between the two components. The photoreduction reaction of CO2 is a multi-step synergistic process. The accumulation of intermediates blocking the active site may be responsible for the decrease in activity (Supplementary Fig. 22).    (Fig. R4f, g).  If I understand correctly, in the present manuscript no STM experiment is performed.  Fig. 4h. Processes Ⅰ and Ⅱ represent the formation of the built-in electric field owing to the difference in the work function, and process Ⅲ represents the separation of the charges following an S-scheme mechanism induced by the built-in electric field. The paper describes an procedural analysis on how and why a recently described all organic heterojunction, PHI-PTI, generates its record high efficiencies in overall water splitting and CO2 reduction. Contrary to previous work which generates its solid-state heterojunction photochemical material by simultaneous cocrystallization and salt melt etching, the present approach is based on independent crystallization and later electrostatic co assembly, which gives a larger structural size enabling to follow photogenerated charges, e.g. by surface potential measurements. Wioth a combination of data, the presence of a S-heterojunction as well as the relative role in oxidation and reduction processes was determined.
This makes the paper interesting and worth the ctation by a very large, very competetive community.
This also brings me to my very important point of criticism dealing with correct citations within the ultracompetetive asian market of artificial photosynthesis.
To make a long analysis short: the manuscript essentially cites one of the two schools, even overscribing priority of the other school by citing later secondary papers of the first school. Being not involved in this specific fight for leadership, I find it nevertheless non acceptable. I seriously and kindly ask the authors to consider the rules of fairness and respect. It is to my opinion indeed a very good, careful paper, and unbalanced recitation of the market can only ruin that impression.
Response: It is a great honor to have this work rated so highly by the reviewer, which makes the joint efforts of all the authors over the past two years meaningful.
Regarding the issue of correct citation raised by the reviewers, we must admit that the early studies on PHI/PTI heterojunctions were indeed overlooked by us and we are very grateful to the reviewers for the reminder. We have carefully The energy barrier of electron transport at the interface between layers will be explained detailly in (3). In addition, a detailed schematic of the atomic arrangement has been added to Supplementary Fig. 5d and e in the supporting information, while the atomic arrangement at the interface has been elucidated in the revised manuscript (page 6, line131-133). And as suggested by the reviewer, we have also added TEM characterisation (Fig. 2).  Reviewer #2 (Remarks to the Author): The authors have fully addressed the remarks made by me and by the other reviewers,